i) Field of the Invention
This invention relates to a calibration standard for calibrating a machine employed in the determination of the somatic cell count of milk, and more especially a shelf-stable standard; the invention also relates to the use of the calibration standard in the determination of the somatic cell count of milk samples.
ii) Description of Prior Art
The production of high quality milk is an objective of the dairy industry. By providing special care and close monitoring of the health status of their dairy cattle, farmers hope to reduce losses due to mastitis. Most commonly, subclinical mastitis caused by bacterial infections leads to a decrease in milk production. Early detection of such infections, is the cornerstone of udder health and has been shown to improve milk production and reduce production costs.
Mastitis occurs when bacteria enter through the streak canal of the teat and move into the udder where they multiply. The onset of mastitis stimulates the cows immune system, with the response of leucocytes migrating into the milk. Leucocytes, also referred to as somatic cells when found in milk, increase in number in milk in relation to the severity of mammary infection. Therefore the ability to accurately enumerate somatic cells in milk serves as an important method to detect mastitis before severe damage can occur to the cow's udder. The methods used to detect somatic cells in milk range from cow side testing to automated cell counters. However, the direct enumeration of somatic cells is usually employed by farm managers in milk recording programs or the dairy food industry for quality control purpose.
One of the services that Dairy Herd Analysis Centers provide to dairy farmers of the world is somatic cell counting of milk for the early detection of mastitis. There are two predominant types of machines which serve this function, the Coulter counter and the Fossomatic type cell counter, and the more recent Bentley Soma, referred to above.
Automated cell counting systems measure fluorescent light emission that occurs as stained somatic cells are observed by an electronic micro-optical system or when unstained somatic cells move through an electromagnetic field and are counted.
Typical of commercial machines which measure fluorescent light emission are the Fossomatic (Trade Mark) and Bentley Soma (Trade Mark) machines. Typical of machines which measure cells moving through an electromagnetic field is a Coulter (Trade Mark) counter.
Direct microscopic count can be achieved by examination of milk somatic cells under a microscope, the cells being fixed and stained to facilitate counting.
When employing these machines it is necessary to employ a calibration standard, and the standards widely used are cells from milk. The cell count of these standards is achieved by the method of direct microscopic determination, which counting method is tedious, time consuming and costly. Additionally, calibration standards derived from milk are of low stability and have a short shelf life.
Calibrating standards based on microbeads have been proposed for the alignment and calibration of flow cytometers and fluorescent microscopes. Representative Patents in this field are U.S. Pat. Nos. 4,867,908; 4,857,451; 5,073,497; 4,609,689; 5,073,489; 4,774,189; 4,714,682 and 4,868,126. These calibrating standards are employed under flow conditions which are such that the beads remain in suspension. These calibrating standards are not suitable as milk calibration standards employing the commercially available machines described hereinbefore. In particular the commercially available machines do no employ the standard under flow conditions, and under static or non-flow conditions the microbeads in these calibrating standards for flow cytometers and fluorescent microscopes, settle from suspension and distort the results received. Surprisingly, agitation to resuspend the microbeads does not overcome the problem. Although available commercial machines, such as the Fossomatic and Bentley machines have some agitating capability it does not appear to be effective in re-suspending beads.